One method of generating a random number on a digital integrated circuit is to use a number of ring oscillators, (typically an odd number of digital inverters connected in a loop), with their outputs latched and those latched values having an XOR function performed on them. If the individual ring oscillators have different phases the output cannot be predicted because the capture of the current output state (0 or 1) by the latches will include capturing of some transitioning edges. The results of those that are transitioning will not be predictable due to noise. Prior art designs have relied on random circuit variations unintentionally created during manufacturing to ensure the ring oscillators have slightly different phases between each of the many ring oscillators in the design.
The individual ring oscillators in the prior art typically have circuitry to enable (or inhibit) their oscillation. There is a possibility that when they are first started the rings may be similar enough that the latching and XORing function will catch a consistent set of all zeros or all ones, or behave in a predictable manner, delivering a predictable output stream for some period of time.